1. Field of the Invention
The invention relates to the structure of a power supply system and particularly to a controller for a system that includes two parallel-connected power supplies and that converts voltage through PWM control.
2. Description of Related Art
Generally, there are many voltage converters that are used to convert the voltage of a battery by turning on or off switching elements through pulse width modulation control. In recent years, there is suggested a power supply system that uses a voltage converter composed of four switching elements and including a series mode and a parallel mode (see, for example, Japanese Patent Application Publication No. 2013-13234 (JP 2013-13234 A). In the series mode, voltage is converted in a state where two batteries are connected in series with each other by a combination of on/off operations of the switching elements. In the parallel mode, voltage is converted in a state where the two batteries are connected in parallel with each other by a combination of on/off operations of the switching elements. When the voltage converter that is used in this kind of power supply system is operated in the parallel mode, voltage conversion control over the batteries is executed by turning on or off the switching elements in accordance with two pulse width modulation control signals corresponding to the two batteries. There is suggested in JP 2013-13234 A that, when this kind of voltage converter is operated in the parallel mode, a loss of the switching elements is suppressed by changing the phases of the two pulse width modulation control signals.
Incidentally, there is known that, in an existing voltage converter that converts the voltage of a battery through on/off operations of switching elements, an electromagnetic sound arises because of the on/off operations of the switching elements. Because the electromagnetic sound is cacophonous noise when the electromagnetic sound falls within the human audible range, there has been studied, for example, a method, or the like, that reduces noise by raising the carrier frequency of pulse width modulation control to raise the frequency of the electromagnetic sound beyond the audible range. However, noise reduction for the power supply system that uses the voltage converter that executes voltage conversion control over the two batteries by using the two pulse width modulation control signals as described in JP 2013-13234 A has not been sufficiently studied.